Bone is broken down by osteoclasts, and rebuilt by osteoblasts, both of which communicate through cytokine (TGF-β, IGF) signalling.
Ossification (or osteogenesis) is the process of laying down new bone material by cells called osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in the formation of normal, healthy bone tissue:Intramembranous ossification is the direct laying down of bone into the primitive connective tissue (mesenchyme), while endochondral ossification involves cartilage as a precursor. In fracture healing, endochondral osteogenesis is the most commonly occurring process, for example in fractures of long bones treated by plaster of Paris, whereas fractures treated by open reduction and stabilization by metal plate and screws may heal by intramembranous osteogenesis.
Heterotopic ossification is a process resulting in the formation of bone tissue that is often atypical, at an extraskeletal location. Calcification is often confused with ossification. Calcification is synonymous with the formation of calcium-based salts and crystals within cells and tissue. It is a process that occurs during ossification, but not vice versa.
The exact mechanisms by which bone development is triggered remains unclear, but it involves growth factors and cytokines in some way.
Parts of this article (those related to PMID 24292720) are outdated. Please update this article to reflect recent events or newly available information.(December 2013)
Intramembranous ossification forms the flat bones of the skull, clavicle and mandible. A detailed description of the process can be found at the page for Intramembranous Ossification.
Endochondral ossification is the formation of long bones and other bones. This requires a hyaline cartilage precursor. There are two centres of ossification for endochondral ossification.
The primary centre
Bones first appear in the diaphysis (middle of shaft), Chondrocytes multiply and form trebeculae. (spongy bone). Cartilage is progressively eroded and replaced by hardened bone, extending towards the epiphysis. Perichondrium layer surrounding the cartilage forms the periosteum, which generates osteogenic cells that then goes on to make a collar that encirclces the outside of the bone and remodels the medullary cavity on the inside. NOTE: most other bones (e.g. vertebrae) also have primary ossification centres and bone is laid down in a similar manner. The nutrient artery enters via the nutrient foramen from a small opening in the diaphysis. It invades the primary centre of ossification, bringing osteogenic cells (osteoblasts on the outside, osteoclasts on the inside.) NOTE: canal of nutrient foramen directed away from growing end when one end grows more than the other. When bone grows same rate at both ends, nutrient artery is horizontal to the bone.)
The secondary centres generally appear at the epiphysis. Secondary ossification mostly occurs after birth (except for distal femur and proximal tibia which occurs during foetal development). The epiphyseal arteries and osteogenic cells invade the epiphysis, depositing osteoblasts and osteoclasts which erode the cartilage and build bone. This occurs at both ends of long bones but only one end of digits and ribs.
Several hypotheses have been proposed for how bone evolved as a structural element in vertebrates. One hypothesis is that bone developed from tissues that evolved to store minerals. Specifically, calcium-based minerals were stored in cartilage and bone was an exaptation development from this calcified cartilage. However, other possibilities include bony tissue evolving as an osmotic barrier, or as a protective structure.